High voltages required for an application are often obtained by connecting multiple smaller voltage sources in series. For instance, an electric or hybrid automobile design may call for twenty 42 volt batteries to be connected in series in order to satisfy the 800 volt minimum power source required by the vehicle. This use of multiple smaller voltage sources over one larger source is preferable as there is less concern of safety in the handling of smaller voltages.
To assure further safety, relay switches are often utilized in applications like that described above. When high voltage is needed, relay switches in-between each smaller voltage source can be closed, thereby closing the circuit and placing each smaller voltage source in series with one another. When high voltage is no longer needed, i.e. an electric car turned off, the relay switches are opened. In this manner, high voltage is present only when necessary while individual small voltages are present at all other times, thereby increasing the overall safety of the system.
In typical systems where relay switches connect multiple voltage sources in series, each relay must be rated to handle the maximum voltage of the system. For instance, in the electric automobile example presented above, each relay would need to be capable of handling 840 volts. This is because perfect timing in relay activation and deactivation is unobtainable. Even though all the coils of the relays are energized and de-energized by the same control unit, the contacts of each relay will not close and open at the same time due to inherent variation and tolerances within each relay switch. As a result, even in a controlled situation, one relay contact will close later or open earlier than the rest. The relay contact that closes last or opens first sees the highest voltage created by the summation of all the smaller voltage sources.
This situation similarly exists in uncontrolled conditions where the relay switches are unavoidably de-energized, for instance, when there is a loose battery terminal, or an inertial switch designed to cut power in an electric vehicle in the event of a crash. In these events, one relay contact will open earlier than the others, and thus see the full voltage of the system.
Based on the above, every relay in a typical multiple voltage source system needs to be rated to handle the maximum voltage created through the summation of all the individual voltage sources. Continuing on with the electric vehicle example above, although only 42 volt battery packs are utilized, each relay must be capable of handling 840 volts as any one of the relays could be the one to open first or close last, and thus see the full voltage created through the summation of all the battery packs. This results in significant expense. Therefore, the inventors hereof have recognized the need for a new circuit and method for controlling multiple relays, thereby allowing the use of lower voltage rated relays.